<p>The present study deals with the relation between Neogene uplift and volcanism of the SE Colombian Andes and sedimentation processes in the Upper Magdalena Valley. The southernmost part of the Upper Magdalena Valley, the S. Neiva Basin, is located between latitudes 2°08'-2°31 N and longitudes 75°22'-75°37'W. To the east, the basin borders the Garzón Massif, the southernmost part of the Eastern Cordillera. To the west, the basin borders the terrain of a fold and thrust belt that was active from the Eocene to the L.Miocene. The Central Cordillera is located W of the fold and thrust belt.<p>Three subinvestigations were carried out. A fission track study was performed on rocks from the Garzón Massif. Sedimentological, stratigraphical and geochronological investigations were accomplished in the S. Neiva Basin as well as in the area of the fold and thrust belt and part of the Central Cordillera. For the sake of clarity, these subinvestigations are discussed in different parts of the thesis.<p>In part 1 (chapter I) the goals of the investigation are presented. Furthermore, information is given on the regional geology and tectonic setting and on published literature, aerial photographs and topographical sheets, used during the present study.<p>Part 2 (chapter II) discusses the results of the fission track study of the Precambrian Garzón Massif. Fission track age determinations of apatites, zircons and sphenes revealed that three major uplift and cooling phases occurred. Around 900 Ma ago, an orogeny resulted in ≥10 km uplift. Some 100 Ma ago block faulting and differential uplift took place and between ≥12 Ma ago and the present time, the Garzón Massif was uplifted some 6.5 km.<p>Part 3 (chapters III to VIII) deals with the Neogene deposits in the S. Neiva Basin. In chapter III the stratigraphy of the different rock units is discussed. Emphasis is on the Honda Formation, the Gigante Formation and the informal Las Vueltas formation.<p>In chapter IV the age of these formations is given, established by means of K-Ar and fission track age determinations. The age of the Honda Formation ranges approximately from ≥16 Ma to 10-9 Ma; the age of the Gigante Formation from 10-9 Ma to ≤6 Ma. The age of the volcaniclastic middle member of the Gigante Formation could be more accurately determined and ranges from 8 Ma to 6.4 Ma. The age of the Las Vueltas formation could not be established, because the rocks do not contain volcanic material. On the grounds of the age of the Gigante Formation and a K-Ar age of ±1 Ma for the oldest Quaternary terrace, the formation is thought to have an Upper Miocene to Pliocene age.<p>Chapter V is dedicated to the petrology of the three formations mentioned above. Pebble and point counts show that the sandstones and conglomerates of the Honda Formation and the lower two members of the Gigante Formation have a western provenance. The upper member of the Gigante Formation, however, shows strong admixing of material with an eastern provenance and the Las Vueltas formation consists entirely of erosional products from the Garzón Massif. These facts suggest that the first Neogene uplift phase of the massif ≥12 Ma ago had little influence on the deposits of the Honda Formation. They also suggest that a second uplift phase took place, which started during deposition of the upper member of the Gigante Formation and culminated during deposition of the Las Vueltas formation.<p>In chapter VI the sedimentological characteristics of the Honda Formation are discussed. It appears that during periods of active uplift or volcanism, sediments were deposited by different alluvial fan systems and volcanic aprons, prograding on a broad alluvial plain. During intervening periods of greater volcanic and tectonic quiescence, the alluvial fans or volcanic aprons became much reduced in size, and sediments were laid down by braided and meandering river systems, flowing toward the east on the alluvial plain.<p>In chapter VII the sedimentological characteristics of the Gigante Formation are treated. The lower and upper members of the formation were deposited by an ancestral Magdalena River, flowing toward the N. During deposition of the middle member, enormous quantities of volcaniclastics flooded the basin, forcing the paleo-Magdalena River to a more eastward position.<p>Chapter VIII deals with the tectonics of the S. Neiva Basin. The second Neogene uplift pulse of the Garzón Massif, which started around 6.4 Ma ago, coincided with strong SE-NW compression, leading to folding and faulting of the basinal sediments. It is likely that deposition of the upper unit of the Las Vueltas formation resulted from strike-slip movements along the faults in the massif. On the basis of the approximate age of the sediments that underwent wrench-faulting, it is concluded that these movements began in the Pliocene. Wrench faulting appears to have had little influence on the faults within the S. Neiva Basin.<p>Part 4 of the thesis (chapters IX to XI) is dedicated to deposits outside the S.Neiva Basin, which were investigated during the present study. These deposits comprise two small outcrops of fluvial and volcaniclastic sediments of Gigante age (chapter IX) as well as Pliocene to Quaternary volcaniclastic sediments of the fold and thrust belt area and the Central Cordillera (chapters X and XI).<p>In chapter IX it is shown that the geological history of the S.Neiva Basin and the Garzón Massif is compatible with the geological history of the Suaza Valley, further to the S, and the Miocene history of the fold and thrust belt area.<p>Chapter X deals with the Pliocene volcaniclastic deposits in the fold and thrust belt area and the Central Cordillera. These deposits comprise the informal fluvio-volcanic El Carmen formation, with an age of ≥3.3 Ma, and the ignimbrites of the Guacacallo Formation, with an age of 2.5±0.2 Ma. The stratigraphical relation between the ignimbrites and the Gigante Formation further to the E, in the S. Neiva Basin, suggests that the basin mainly underwent erosion in the period between *6 Ma and 2.5±0.2 Ma.<p>Chapter XI treats the volcaniclastic terraces along the Páez River and the downstream part of the La Plata River. The deposits resulted from two separate influxes of volcaniclastic material, probably from the Nevado del Huila. The older deposits have an age of ±1 Ma; the younger deposits are dated at <1 Ma. After deposition, erosional terraces were formed in the sediments. The terraced deposits are lateral equivalents of similar terraces along the part of the Magdalena River, located between Gigante and El Hobo.<p>In part 5 (chapter XII) a synthesis of the data from the previous chapters is given, and the geological histories of the different areas are integrated in a single model. Furthermore, uplift of the Garzón Massif is discussed in the light of the plate tectonic setting. Neogene uplift phases of the Garzón Massif coincided with cessation of activity of the Central Cordillera volcanic arc. Volcanism was resumed during periods of tectonic quiescence. The fact that Laramide-style uplift was contemporaneous with absence of volcanic activity, suggests that low-angle subduction of part of the Nazca Plate took place. Beside low-angle subduction, however, crustal predisposition to a certain style of uplift probably played a role in determining the style of mountain building. Thus it is likely that Laramide-style uplift of the massif was also influenced by the presence of inherited zones of structural weakness.
|Qualification||Doctor of Philosophy|
|Award date||26 Nov 1991|
|Place of Publication||S.l.|
|Publication status||Published - 1991|
- water erosion
- volcanic activity